A fortification, also called a fort, fortress, fastness or stronghold, is a military construction designed for the defense of territories in warfare and used to establish rule in a region during peacetime. The cited article says the term is derived from the Latin fortis, strong, and facere, to make, that from very early history to modern times defensive walls have often been necessary for cities to survive invasion and conquest, that some settlements of the Indus Valley Civilization were the first small cities to be fortified, and that in ancient Greece large cyclopean stone walls had been built. Buildings recorded in the atlas as forts and fortresses are grouped under this concept.